Photocatalyst and process for producing the same

ABSTRACT

A photocatalyst which has high catalytic activity, is nontoxic, has a long life, can utilize visible light as it is for photocatalytic reactions, and is useful especially for hydrogen generation; and a process for producing the same. The photocatalyst comprises cadmium sulfide, has a capsule structure, wherein platinum is supported thereto. It can be obtainable by bubbling H 2 S gas into a liquid to which particles of cadmium oxide have been added.

FIELD OF THE INVENTION

The present invention relates to a photocatalyst. More particularly, theinvention relates to a photocatalyst which has high catalytic activity,is nontoxic, has a long life, can utilize visible light as it is forphotocatalytic reactions, and is useful especially for hydrogengeneration, etc., and to a process for producing the same.

BACKGROUND ART

To obtain chemical energy from solar energy, i.e., to utilize hydrogenenergy, which is infinite and clean, is a dream of human beings. Energyissues in the 21st century and the global warming effect of carbondioxide produced by fossil energy as well as environmental pollutionincluding acid rain can be resolved by putting that energy intopractical use.

The Honda-Fujishima effect published in A. Fujishima et. al., Nature,238, 37(1972) was the first attempt which showed that water can bedecomposed into oxygen and hydrogen with light energy. Thereafter, inthe days when the oil crisis aroused questions all over the world, manyinvestigations were actively made on techniques for converting lightenergy into chemical energy based on that principle. However, noimprovement in the efficiency of light energy conversion in the visiblelight region has been made so far. A result of the active investigationsmade in the days of from 1980 to 1990 is that it was demonstrated thatthe electrons and holes generated by optical excitation recombine beforereaching reaction sites for water decomposition and this recombinationgoverns the efficiency of conversion. It was attempted to utilize anintercalation compound in order to separate reaction sites in view ofthat conclusion (S. Ikeda et. al., J. Mater. Res., 13, 852(1998)).Although the efficiency of conversion has been gradually improved, asatisfactory conversion efficiency in the visible light region has notyet been attained. This is because complete separation of reactionsites, i.e., separation between electrons and holes, has not beenattained.

Simultaneously with the investigations described above, investigationswere made on a reaction system for yielding hydrogen by utilizing lightabsorption by ions in a solution. It was reported in J. Jortner, et.al., J. Phys. Chem., 68, 247 (1964) and K. Hara, et. al., J. Photochem.PhotoBiolo. A128, 27 (1999) that hydrogen is yielded at a high quantumefficiency in an acidic solution containing iodine ions and in an alkalisolution containing sulfur ions, respectively. However, all thesereactions are possible with high-energy ultraviolet light having awavelength of 250 nm or shorter.

On the other hand, since photocatalysts have the property ofaccelerating various chemical reactions such as, e.g., the decompositionof environmental pollutants, malodorous ingredients/various bacteria, orthe like, they have come to be practically used in applications such astiles having antibacterial activity and antibacterial/deodorizingfilters for air cleaners. Furthermore, it is possible to cause aphotocatalyst to act on a harmful substance to obtain a useful chemicalsubstance therefrom. For example, photocatalysts are expected to beapplied to a crude oil desulfurization step.

In the step of desulfurizing crude oil which is generally conductedpresently, heavy naphtha is subjected to hydrofining during crude oildistillation to recover all the sulfur ingredients contained in thecrude oil as hydrogen sulfide. This hydrogen sulfide is recovered afteroxidation of sulfur through the process called Claus process. The Clausprocess is a process in which one-third of the hydrogen sulfide isoxidized into sulfurous acid gas and this gas is reacted with theremaining hydrogen sulfide to obtain elemental sulfur.

This process necessitates an enormous amount of energy because of notonly the catalytic reaction of sulfurous acid gas with hydrogen sulfidebut also repetitions of heating and condensation. It further hasproblems, for example, that the management of sulfurous acid gas iscostly.

If a method which comprises adding a photocatalyst to an aqueous alkalisolution containing hydrogen sulfide dissolved therein, irradiating theresultant mixture with light to thereby cause the photocatalyst toabsorb the light energy of the radiation and generate free electrons andfree holes, and oxidizing/reducing the aqueous alkali solutioncontaining dissolved hydrogen sulfide by the free electrons and holes toobtain hydrogen and sulfur, i.e., a method for decomposing hydrogensulfide with a photocatalyst to yield hydrogen and sulfur, can be put topractical use, it becomes possible to decompose hydrogen sulfide as aharmful substance with a smaller amount of energy to produce hydrogenand sulfur as useful substances. Namely, this technique contributes tothe resolution of environmental problems and enables the usefulsubstances to be produced at low cost.

However, the photocatalysts proposed so far have had the followingproblems which should be overcome. First, the catalytic activity is low.Secondary, the photocatalysts are toxic. Although the photocatalystsgenerate free electrons and free holes upon irradiation with light, theprobability that the free electrons recombine with the free holes ishigh. Furthermore, the probability that chemical substances formedthrough decomposition by oxidation/reduction reactions recombine witheach other and return to the original compound is also high. Because ofthese, the catalytic activity is low.

Thirdly, the catalysts have a short life. The prior-art photocatalystshave the following problem concerning photodissolution. Although thephotocatalysts generate free electrons and free holes upon irradiationwith light, the catalysts themselves are oxidized/reduced besides thetarget chemical substance because of the high susceptibility tooxidation/reduction reactions of these electrons and holes. Thecatalysts thus dissolve away and are deprived of their catalyticactivity.

For overcoming these problems, JP-A-2001-190964 disclosed aphotocatalyst having high catalytic activity, no toxicity, and a longlife. Thus, the three problems described above were eliminated.

However, the photocatalyst disclosed in JP-A-2001-190964 is limited toone comprising ZnS. Since the band gap for ZnS is in the ultravioletregion, it has been impossible to utilize visible light such as, e.g.,sunlight, which is infinite clean energy, as it is for photocatalyticreactions.

DISCLOSURE OF THE INVENTION

Accordingly, an object of the invention is to overcome the drawbacks ofthe related-art techniques described above and provide a photocatalystwhich has high catalytic activity, is nontoxic, has a long life, canutilize visible light as it is for photocatalytic reactions, and isuseful especially for hydrogen generation. Another object of theinvention is to provide a process for producing the photocatalyst.

As a result of intensive investigations, the present inventors havesucceeded in eliminating those problems by employing the followingconstitutions.

Namely, the invention is as follows.

(1) A photocatalyst which comprises cadmium sulfide, has a capsulestructure, wherein platinum is supported thereto.

(2) A process for producing a photocatalyst, which comprises bubblingH₂S gas into a liquid to which particles of cadmium oxide have beenadded.

(3) The process according to the above (2), which comprises supportingplatinum thereafter.

Since the photocatalyst of the invention comprises cadmium sulfide,visible light such as, e.g., sunlight, which is an infinite cleannatural energy, can be utilized as it is for reactions catalyzed by thephotocatalyst. In addition, the photocatalyst is nontoxic and has a longlife.

The photocatalysts of the related art which comprise simple particles ofcadmium sulfide have had an exceedingly low efficiency of light energyconversion.

In contrast, the photocatalyst of the invention has a capsule structurewhich comprises a shell comprising cadmium sulfide and has a void. Thefollowing is thought. In this capsule shell comprising cadmium sulfide,an electric field is present between the outer side and inner side ofthe shell. Because of this, the recombination of free electronsgenerated by irradiation with visible light, e.g., sunlight, with freeholes simultaneously generated by the irradiation is diminished and therecombination of oxidation reaction products with reduction reactionproducts is also diminished. High catalytic activity can hence beobtained.

By the mechanism described above, the photocatalyst of the invention canbe one which has high catalytic activity, is nontoxic, has a long life,and can utilize visible light as it is for photocatalytic reactions andwhich is useful especially for hydrogen generation.

The photocatalyst of the invention will be explained below in detail.

The average particle diameter of the photocatalyst of the invention,which has a capsule structure, is not particularly limited. However, itmay be about 250 nm.

Incidentally, the average particle diameter of the photocatalyst of theinvention, which has a capsule structure, is determined by taking someelectron photomicrographs of from several to tens of catalyst particles,measuring the length of the major axis of each particle image on thephotographs, and averaging the found values.

In the photocatalyst of the invention, platinum is supported thereto forthe purposes of further enhancing its photocatalytic activity andsecuring hydrogen generation sites.

The structure of the photocatalyst of the invention will be explainedbelow.

The photocatalyst according to the invention has a shell comprising acadmium sulfide compound and a void. The shell of the photocatalyst ofthe invention has a stratified structure comprising a layer of ultrafinecadmium sulfide compound particles having a particle diameter of from 1nm to 10 nm. Photocatalysts such as that of the invention are calledstratified photocatalysts. Furthermore, the shell having a stratifiedstructure of the photocatalyst of the invention has a possibility thatit might have a structure in which the proportion of cadmium (Cd)element to sulfur element changes in the layer thickness direction. Itis thought that an electric field is hence present in the layerthickness direction and this diminishes the recombination of freeelectrons generated by irradiation with visible light, e.g., sunlight,with free holes simultaneously generated by the irradiation and therecombination of oxidation reaction products with reduction reactionproducts to thereby give high catalytic activity.

FIG. 1 is a transmission electron photomicrograph of a stratifiedmaterial comprising fine cadmium sulfide compound particles which is oneembodiment of the photocatalyst of the invention; the photomicrographwas taken with irradiation with electron beams.

It can be seen from the photograph that the stratified material has acapsule structure constituted of ultrafine CdS particles of severalnanometers.

Processes for producing the photocatalyst of the invention are notparticularly limited. However, the simplest process is as follows.

H₂S gas is bubbled into a liquid to which particles of cadmium oxide(CdO) having no photocatalytic activity have been added, and the mixtureis then allowed to stand. As a result, ultrafine particles of cadmiumsulfide (CdS) are formed on the surface of the cadmium oxide (CdO)particles. In this operation, cadmium oxide (CdO) is consumed and thiscadmium ingredient forms a layer of ultrafine particles of cadmiumsulfide (CdS). This layer formation occurs from the surface of the CdOparticles. Because of this, the cadmium oxide (CdO) present inside thecapsule structure is dissolved away. The photocatalyst of the inventionhence has a stratified capsule structure having a void inside.

After formation of the photocatalyst having this stratified capsulestructure, platinum serving as hydrogen generation sites is supportedthereto by deposition from, e.g., a solution of chloroplatinic acid.

The cadmium oxide particles having no photocatalytic activity to be usedin the production process described above may be commercial ones, or maybe ones suitably produced.

The particle diameter of the photocatalyst to be produced can besuitably regulated according to the intended use of the catalysisthereof. Examples of methods for regulating the particle diameterthereof include one in which the particle diameter of the cadmium oxideparticles having no photocatalytic activity is regulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron photomicrograph showing the structure of aphotocatalyst of the invention.

FIG. 2 is a view showing the constitution of the apparatus used formeasuring the amount of hydrogen generated.

FIG. 3 is an illustration showing a comparison between a photocatalystof the invention and an existing photocatalyst in hydrogen generationperformance.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be explained below in more detail by reference toExamples, but the scope of the invention should not, of course, beconstrued as being limited by these.

(Photocatalyst Production)

One gram of commercial reagent CdO (manufactured by Kojundo ChemicalCo., Ltd.; particle diameter, 200-300 nm) was added to 50 mL ofdistilled water. H₂S gas was bubbled into the mixture with stirring at aflow rate of about 50 mL/min for 1 hour. Thereafter, the mixture wasallowed to stand for 24 hours and subjected to solid-liquid separation.The solid matter obtained was subjected to an acid treatment withhydrochloric acid and a washing treatment with distilled water and thento a solid-liquid separation. A 250-mg portion of the solid matterobtained was added to 100 mL of 0.965 mM chloroplatinic acid solution.This mixture was irradiated, with stirring, with ultraviolet light for 5minutes using a mercury lamp, and then subjected to solid-liquidseparation.

The solid matter obtained was examined with a transmission electronmicroscope, TEM. As a result, the structure shown in the TEM photographof FIG. 1 was ascertained.

<Evaluation of the Photocatalyst>

The amount of hydrogen generated with the photocatalyst particlesobtained above was measured by the following test method.

(Test Method for Measuring Amount of Hydrogen Generated)

A hundred milligrams of photocatalyst particles are introduced into anapparatus for measuring the amount of hydrogen generated which isconstituted of a buret and other components. Subsequently, 140 mL of 0.1M sodium sulfide solution is introduced into the apparatus for measuringthe amount of hydrogen generated.

The apparatus is irradiated from under the same with an artificialsunlight using a 450-W xenon lamp manufactured by Wakomu Denso K.K., andthe amount of hydrogen generated in every unit time period is measured.

For the purpose of comparison, commercial CdS particles (manufactured byWako Pure Chemical Industries, Ltd.; particle diameter, 0.3 μm) whichneither had the stratified capsule structure in the invention nor hadplatinum were used under the same conditions in a comparativeexperimental example.

FIG. 2 illustrates the apparatus used for the measurement of the amountof hydrogen generated. As shown in FIG. 2, this apparatus is constitutedof: a photoreaction part 1 made of quartz glass; a hydrogendetermination part 2 for determining the hydrogen generated; a solutionreservoir 4 into which an aqueous sodium sulfide solution 3 comes in anamount corresponding to the volume of the hydrogen gas generated tothereby prevent an increase in hydrogen pressure; a 450-W xenon lamp(not shown) for irradiation with an artificial sunlight 5; and areflecting mirror 7 for reflecting the artificial sunlight 5 toirradiate a photocatalyst 6. Prior to the initiation of aphotodecomposition reaction, the whole system is filled with an aqueoussodium sulfide solution 3 and a given amount of the photocatalyst 6 issedimented on the bottom of the photoreaction part 1. The opening 8 forrecovering gas generated is closed, and the 450-W xenon lamp is switchedon. The amount of the hydrogen generated is measured in the hydrogendetermination part 2 at a given interval of irradiation time.

<Results of the Test for Measuring Amount of Hydrogen Generated>

FIG. 3 shows the dependence of the amount of hydrogen generated on thevisible light irradiation time. As apparent from the graphs showing acomparison in this performance, the photocatalyst of Example accordingto the invention gave satisfactory results with a hydrogen generationrate of about 50 mL/h as apparent from FIG. 3. In contrast, the test formeasuring the amount of hydrogen generated with the CdS particles ofComparative Example gave unsatisfactory results.

Furthermore, actual sunlight was used to conduct the same experiment. Asa result, the following experimental results were obtained. Sunlight wascondensed with a lens, reflected with the mirror, and caused to strikeon the reaction vessel from under the same. The quality of the sunlightmeasured with a power meter was 14 Wh. The amount of the hydrogen thusgenerated was about 70 mL/h. The intensity of direct sunlight wassimultaneously measured and was found to be 0.23 W/3.14 cm². Theseresults of the measurements mean that about 3.5 L of hydrogen isobtained per hour with sunlight incident on 1 m².

INDUSTRIAL APPLICABILITY

It can be understood from the explanation given above that thephotocatalyst of the invention has high catalytic activity, is nontoxic,can utilize visible light as it is for photocatalytic reactions, and isuseful for hydrogen generation, etc.

1. A process for producing a photocatalyst which comprises a capsulestructure comprising a cadmium compound shell and a void, said processcomprises bubbling H₂S gas into a liquid to which particles of cadmiumoxide have been added.
 2. The process according to claim 1, whichcomprises supporting platinum thereafter.
 3. The process of claim 1,wherein the capsule structure comprises a cadmium compound shell and avoid inside said shell.
 4. A photocatalyst which comprises (i) a capsulestructure comprising a cadmium compound shell and an internal void indirect communication with an inside surface of said cadmium compoundshell, and (ii) platinum supported on an outside surface of said shell.5. The photocatalyst of claim 4, wherein said cadmium compound shell hasa stratified structure.